In a RF-energized sealed-off diffusion cooled pulsed CO2 gas-discharge laser, if time between pulses becomes short enough, the diffusion cooling can become overwhelmed resulting in a temperature rise in the CO2 lasing gas mixture. This can affect the characteristics of output pulses, particularly the rise and fall of the output pulses. This can lead to an inconsistency in power from pulse to pulse and accordingly variations in average power. A typical lasing-gas mixture includes helium (He), nitrogen (N2) and CO2 in proportions of about 80:10:10.
FIG. 1A and FIG. 1B provide a timing diagram comparing a relationship between an RF pulse (FIG. 1A) energizing a prior-art diffusion cooled CO2 gas-discharge laser and a resulting laser output pulse (FIG. 1B) from the laser. It is assumed that the laser is tuned to operate (lase) at a wavelength of about 10.6 micrometers (μm). The RF pulse is initiated at time t0 and terminated at time t3. As depicted here, the RF pulse has a duration of about 100 microseconds or greater.
Initially there is no laser output as time is required for N2 in the lasing gas mixture to be energized by the RF and then transfer that energy to the CO2 by collision. After a relatively short time, there is an initial gain-spike (power spike) in the lasing gas mixture which drops to almost zero at time t1. Because of the very short duration, the energy in this power-spike is not significant from the process perspective. Following the power-spike, the laser output power rises progressively. This period of rising power is designated τR in FIG. 1B. During this time, the temperature of the gas mixture is rising, and at time t2 the gain (at the 10.6 μm CO2 wavelength for which the laser is tuned) begins to gradually fall as a result of competition between energy transitions for the 10.6 μm wavelength and for another lasing wavelength at about 9.6 μm (for which the laser-resonator is assumed not to be tuned). A detailed description of the physics of the gain-reduction is not necessary for understanding principles of the present invention and is not presented herein. This gradual falling period of the 10.6 μm-power, due to heating of the lasing gas mixture, is designated in FIG. 1B as period τH. At time t3, when the RF pulse is terminated, gain, and accordingly the laser pulse power, falls exponentially toward zero over a fall-time designated in FIG. 1B as period τF.
If the pulse RF-pulse duration is made sufficiently short, the gas-heating effect in an individual laser pulse as depicted in FIG. 1B can be avoided. However, in a train of laser-pulses with a sufficiently short duration between pulses, gas-heating still occurs but the effect is manifest in a different way, discussed below.
FIG. 2A and FIG. 2B provide a timing diagram comparing a relationship between a train of three RF pulses ARF, BRF, and CRF (FIG. 2A) and resulting laser output pulses AL, BL, and CL, respectively. It is assumed, here, that the RF pulses are delivered at a pulse-repetition frequency F that provides a time T between pulses on the order of 0.15 milliseconds (ms). The pulse duration (TP) is assumed to be about 25 microseconds (μs). This pulse duration is sufficiently short that the tH effect of FIG. 1B is avoided. The time between pulses however is sufficiently short that the lasing gas does not return to the temperature at which one pulse is initiated before the next is initiated. A result of this is that the peak power PL2 of pulse BL is less than the peak power PL1 of pulse AL, and the peak power PL3 of pulse CL is less than the peak power PL2 of pulse BL. This peak power reduction from one pulse to the next continues until a steady state gas-heating condition is reached. In the case of the exemplary 0.15 ms between pulses, and for a lasing gas pressure of about 100 Torr, this may not occur until about 7 pulses have been delivered.
FIG. 3 is a graph schematically illustrating measured amplitude ratio (indicated by diamonds) between isolated laser-output pulse pairs as a function of time between pulses in a prior-art diffusion-cooled CO2 laser. The circle indicates that pulse separation time that corresponds to a pulse repetition frequency of 3 kilohertz (kHz). The RF pulse duration (excitation pulses) in this case was about 25 μs, but similar results were obtained with RF pulses having a duration of 50 μs. It can be seen that amplitudes of consecutive pulses become about equal when time between pulses is between about 750 and 1000 microseconds or greater.
For delicate laser machining operations, such as laser drilling of via holes in printed circuit boards (PCBs), maintaining pulse-to-pulse consistency is very important. An onset of pulse inconsistency defines an upper limit to the pulse-repetition rate that can be used, and accordingly a limit to the throughput of the operation. In most cases, the steady-state condition is not an option, because a particular drilling sequence requires require pulses-on-demand at irregular intervals. Accordingly, there is need for a method and apparatus that can maintain pulse-to-pulse consistency of laser output pulses triggered on demand with relatively short intervals therebetween.